The present invention relates to electronic ballasts and a lighting fixture using the same.
Electronic ballasts for powering a discharge lamp such as a fluorescent lamp are well known. On example of a conventional electronic ballast is shown in FIG. 10, and includes a DC converting circuit 1 for converting an output of an AC power source AC into a DC output, an inverter circuit 2 for converting the DC outputted from the DC converting circuit 1 into a high frequency output, a resonant circuit 3 having a series resonant circuit formed of an inductor L2 connected to an output end of the inverter circuit 2 and a capacitor C2, and a discharge lamp La powered by an output of the inverter circuit 2, and a control circuit 4 for controlling operations of the DC converting circuit 1 and the inverter circuit 2.
The DC converting circuit 1 includes a diode bridge DB, a series circuit formed of an inductor L1 and a diode D1, which is connected to an output on a high-voltage side of the diode bridge DB, and a step-up chopper power factor correction (PFC) circuit formed of a switching element Q1 connected between output ends of the diode bridge DB through the inductor L1, and a capacitor C1 connected to the switching element Q1 in parallel through the diode D1. In the DC converting circuit 1, the diode bridge DB rectifies an output voltage from the AC power source AC, and a DC control circuit 4a described later turns on/off the switching element Q1 through the driver 4b to convert the output voltage from the diode bridge DB into a desired DC voltage. The DC output voltage is smoothed by the capacitor C1. Furthermore, the power factor of an input current is corrected by bringing the input current to close to a sinusoidal current.
The inverter circuit 2 includes switching elements Q2, Q3 connected in series between across capacitor C1, and a capacitor C3, one end of which is connected to a connection center point of the switching elements Q2, Q3. A microcontroller 4c described later alternately turns on/off the switching elements Q2, Q3 through the driver 4d to convert the output voltage from the DC converting circuit 1 into a high-frequency AC voltage and feeds the AC voltage to a discharge lamp La of the resonant circuit 3. The capacitor C3 blocks DC components from the AC voltage and is charged at turn-on of the switching element Q2 to act as a power source for supplying power to the discharge lamp La at turn-on of the switching element Q3.
The resonant circuit 3 is a series circuit formed of an inductor L2 and a capacitor C2, which is connected between the other end of the capacitor C3 and a low-voltage side of the series circuit including the switching elements Q2, Q3, and the discharge lamp La connected to the capacitor C2 in parallel. The capacitor C2 is a preheating capacitor and along with the current-limiting inductor L2, constitutes a series resonant circuit.
The control circuit 4 includes the DC control circuit 4a, a driver 4b for turning on/off the switching element Q1 according to an output signal of the DC control circuit 4a, the microcontroller 4c and a driver 4d for turning on/off the switching elements Q2, Q3 according to an output signal of the microprocessor 4c. 
There is provided in the electronic ballast of FIG. 10 a feedback circuit in the control circuit 4 to control power supplied to the discharge lamp La to be substantially constant by detecting a signal in proportional to the power supplied to the discharge lamp La, comparing the detected value with a reference value and performing feedback control.
FIG. 11(a) shows the frequency response characteristic of the resonant circuit 3 in the example of FIG. 10. A resonant curve Sa during non-lighting of the discharge lamp La is determined mainly by the inductor L2 and the capacitor C2. The resonant curve Sb obtained during lighting of the discharge lamp La is determined by the impedance of the discharge lamp La in addition to the inductor L2 and the capacitor C2.
When a main power source is turned on, the inverter circuit 2 operates with an operating frequency at which such voltage that does not ignite the discharge lamp La is applied to the discharge lamp La to preheat filaments F1, F2 at both ends of the discharge lamp La. The inverter operating frequency is changed so that a voltage sufficiently large to start the discharge lamp La is applied to the discharge lamp La. After lamp ignition, by changing the operating frequency to a frequency according to a dimming level, the discharge lamp La is normally lighted. Accordingly, in prior preheating and starting before the discharge lamp La is ignited, the frequency characteristic of the resonant circuit 3 follows the resonant curve Sa during non-lighting, and after lighting of the discharge lamp La, the frequency characteristic of the resonant circuit 3 follows the resonant curve Sb.
The variations in the resonant curve Sa during non-lighting and the resonant curve Sb in lighting occur due to variations in components of the inductor L2 and the capacitor C3. For example, as shown in FIG. 11(b), given that the operating frequency of the inverter circuit 2 in lighting is f1, variations shown at “d” occur in the resonant curve Sb in lighting due to variations in the components of the inductor L2 and the capacitor C2, causing output variations shown at “a”. Frequency variations shown at “c” also occur in the operating frequency of the inverter circuit 2 due to variations in components forming the control circuit 4. For example, given that the operating frequency varies from f3 to f2 (f3<f1<f2), the output varies according to b″. The above-mentioned variations, as shown in FIG. 11(c), also apply to variations in non-lighting, that is, during preheating and starting.
To prevent the above-mentioned variations, for example, a variable resistor is generally used as a resistor for setting the operating frequency so as to perform a fine adjustment. However, when fine adjustment is performed using the variable resistor, an adjustment process using the variable resistor is required in a manufacturing process of the electronic ballast, disadvantageously resulting in lowering of productivity.
Thus, in the above-mentioned example, variations in the operating frequency are eliminated by setting the operating frequency by use of the microprocessor 4c in the circuit for controlling operations of the switching elements Q2, Q3. In a device described in Japanese Unexamined Patent Publication No. 2001-326089, a detecting circuit (not shown) for detecting, for example, a voltage V1 across the discharge lamp La and a current supplied to the discharge lamp La is provided, an output of the detecting circuit is inputted to a terminal having an A/D converting function in the microprocessor 4c, and a suitable operating frequency is selected based on the inputted value to drive the switching elements Q2, Q3. In this manner, variations in the output are narrowed and the electronic ballast can be manufactured without using the variable resistor.
However, in the above-mentioned example, in the case where the feedback circuit as described in Japanese Unexamined Patent Publication No. 2001-326089 is provided, there have been problems. When a delay time of the feedback circuit is set in accordance with the resonant frequency in lighting, in adjusting the resonant frequency in preheating and starting, the resonant frequency in preheating and starting becomes higher than the resonant frequency in lighting and thus, the delay time with respect to the resonant frequency becomes longer. Moreover, for frequency characteristics in preheating and starting, since the voltage applied to the discharge lamp La changes sharply in response to change in the frequency, feedback control of the feedback circuit cannot keep up with the change, and as shown in FIG. 12, since amplitude of the voltage applied to the discharge lamp La varies, the resonant frequency in preheating and starting cannot be adjusted.
In adjusting the resonant frequency in preheating and starting, since the voltage applied to the discharge lamp La changes sharply relative to change in frequency, when the operating frequency falls below a frequency lower than an adjustment range in the adjustment process, a high voltage occurs. For this reason, in consideration of stress applied on components of the circuit due to application of the high voltage, it is necessary to use more robust and more expensive components.